This invention relates to a pressure holding valve used in e.g. a vehicle brake device.
The vehicle brake device proposed by the present applicant in JP patent application 2001-14903 is shown in FIG. 4.
In the figure, a hydraulic pressure control device 1, in response to the depressing force applied to a brake pedal 3, amplifies brake hydraulic pressure supplied from a high-pressure source 2 by means of a proportional pressure control valve 4 and feeds it to wheel brakes 5 in the first line. The high-pressure source 2 has a motor-driven pump 2a and a pressure accumulator 2b. 
Between the wheel brakes 5 in the first line and wheel brakes 6 in the second line, a copy valve 7 is provided. When the first line is pressurized, a piston 7a of the copy valve 7 is pushed rightwardly in the figure. In the initial stage of its travel, an input port 7b of the copy valve 7 will be closed, so that the wheel brakes 6 in the second line are also pressurized.
The proportional pressure control valve 4 has a spool valve 4a which receives introduced hydraulic pressure on its pressure-receiving surfaces having a difference in area by provision of a pin 4f. The spool valve 4a moves to a balance point where the sum of the thrust by hydraulic pressure and the force of a spring 4b balances relative to the force applied from the brake pedal 3, thereby changing over the connection of an output port 4d to an input port 4c and a discharge port 4e and adjusting the degree of opening of passages therebetween.
The proportional pressure control valve 4 includes a solenoid 8 that applies leftward (in the figure) magnetic attraction force to the spool valve 4a, and a permanent magnet 9 for applying magnetic repulsion to the spool valve 4a by supplying a current in a reverse direction to a coil 8a of the solenoid 8. Thus, not only control based on the will of the driver, but also control that is completely independent of the will of the driver and based on commands from an electronic control device (not shown) is possible.
Numeral 10 in FIG. 4 is a hydraulic pressure generator (master cylinder) provided as a failsafe means in case the high-pressure source 2 or the first line fails. It generates hydraulic pressure corresponding to the brake pedal depressing force by compressing brake fluid in a fluid chamber 10a with a first piston 10b. 
If the hydraulic pressure adjusted by the proportional pressure control valve 4 is higher than the hydraulic pressure generated in the fluid chamber 10a, the input port 7b of the copy valve 7 is closed. Thus brake fluid from the fluid chamber 10a has nowhere to go. Thus, the operating amount applied to the first piston 10b from the brake pedal 3 is transmitted to a second piston 10c as it is. This force is transmitted via a spring 11 to the spool valve 4a to perform position control of the spool valve.
On the other hand, if the high pressure source 2 or the first line fails and no hydraulic pressure is generated in the first line, the input port 7b of the copy valve 7 is left open, so that hydraulic pressure generated in the fluid chamber 10a flows to the wheel brakes 6 in the second line. This assures that minimum braking force is supplied even if the first line fails.
If the wheel brake pressure is controlled to be lower than the will of the driver e.g. for regenerative braking, the hydraulic pressure adjusted by the proportional pressure control valve 4 may be lower than the pressure generated in the fluid chamber 10a. A relief valve 12 is provided to keep the input port 7b of the copy valve 7 from opening at this time. A check valve 13 permits return of fluid from downstream, bypassing the relief valve 12. A check valve 14 is provided for replenishment of fluid from a reservoir 15 if there is a possibility of the fluid amount in the copy valve 7 becoming insufficient due to increase in the fluid amount in the brakes in the second line e.g. due to wear of the pads.
The brake device of FIG. 4 holds hydraulic pressure accumulated in the high-pressure source with the proportional pressure control valve 4 while the ignition switch of the vehicle is off. But since the spool valve has a sliding clearance around the spool and minute leak occurs therefrom, if the ignition switch is off for a long time, hydraulic pressure accumulated in the high-pressure source may become lower than the desired value.
Such lowered pressure recovers as the vehicle is restarted and the pump 2a is driven. Thus, any special countermeasure is not necessarily needed. But it is needless to say that it is the best measure to prevent the pressure in the high-pressure source from decreasing to hold it to the desired level.
As a measure of holding pressure, it is conceivable to provide a solenoid valve which closes when the ignition switch is turned off, in a fluid line between the high-pressure source and the proportional pressure control valve. But this method is not preferable because it requires a new countermeasure against failure of the solenoid valve and electric lines.
An object of this invention is to prevent pressure drop of the high-pressure source due to leak from around the spool, which occurs if a pressure adjusting valve having a spool is used, without using a solenoid valve.
According to this invention, there is provided a pressure holding valve comprising a housing provided with a valve chamber having a valve seat, a small-diameter bore communicating with the valve chamber, a large-diameter bore communicating with the small-diameter bore, a first port opening to the center of the valve seat, a second port communicating with the first port through the valve chamber, and a third port communicating with the large-diameter bore,
a piston valve having a valve head mounted in the valve chamber, a small-diameter portion slidably mounted in the small-diameter bore, and a large-diameter piston mounted in the large-diameter bore to partition the large-diameter bore into a front chamber and a rear chamber, the rear chamber being at the atmospheric pressure,
a seal member liquid-tightly sealing around the small-diameter portion of the piston valve in the small-diameter bore, and
a valve closing spring for biasing the piston valve to press the valve head against the valve seat to close the pressure holding valve,
wherein if the hydraulic pressure at the first port drops to a predetermined value, the piston valve is closed by the force of the spring to shut off communication between the first port and the second port, and with the communication shut off, when hydraulic pressure is introduced from the third port into the front chamber of the large-diameter bore, the thrust of the piston valve overcomes the force of the spring, so that the first port and the second port communicate with each other.
If this pressure holding valve is provided with a fourth port in the housing and the rear chamber of the large-diameter bore is connected to a reservoir through the fourth port, the rear chamber of the large-diameter bore is filled with fluid introduced from the reservoir, so that the rear face of the piston and the inner surface of the large-diameter bore will not be exposed to the atmosphere.
There is also provided a vehicle braking device wherein the pressure holding valve is arranged between a high-pressure source having a pressure accumulator and a pressure adjusting valve for adjusting the pressure by displacement of a spool with the first port of the pressure holding valve connected to the high-pressure source, the second port to the pressure-adjusting valve, and the third port to the master cylinder.
While the hydraulic pressure at the first port exceeds a predetermined value, the thrust of the piston valve under the hydraulic pressure overcomes the force of the spring, so that the valve-open state is maintained. This allows free movement of hydraulic pressure between the first port and the second port.
While no hydraulic pressure is introduced into the front chamber of the large-diameter bore from the third port, if the hydraulic pressure at the first port drops to a predetermined value, it is closed by the force of the spring. This prevents release of hydraulic pressure to the second port, thus holding the pressure at the first port.
Further, when hydraulic pressure is introduced into the front chamber of the large-diameter bore from the third port while the valve is closed, thrust under the hydraulic pressure overcomes the force of the spring, so that the valve opens again. This permits free movement of hydraulic pressure between the first port and the second port.
Thus, by employing the pressure holding valve of this invention in a brake device as shown in FIG. 4, it becomes possible to hold the pressure of the high-pressure source without affecting the operation of the device whatsoever.
Since the pressure holding valve of this invention is a pressure-responsive valve, it is superior to solenoid valves in reliability.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: